1. Field of the Invention
The present invention relates generally to manufacturing processes involving the coating of substrates. More particularly, the present invention relates to apparatuses and methods used for sputtering thin films.
2. Description of the Related Art
Various manufacturing processes involve the deposition or coating of multiple layers of materials on a substrate by sputtering. A basic sputtering operation includes bombarding a target material with ions to release atoms from the surface of the target. The released atoms are directed towards the substrate so that they become deposited on the surface of the substrate. To build up the desired multiple layers of different materials, the sputtering operation is repeated with a previously coated substrate, using targets of different materials in each sputtering operation.
To increase production yield, pallets are used to support and transport the substrates through the various sputtering operations, with each pallet being designed to carry a plurality of substrates arranged in an array. To obtain desirable films characteristics as well as improve yields, a bias voltage is often applied to the substrates during the sputtering process. While using a pallet may be more efficient than processing the substrates individually, it also means that all the substrates on the same pallet are set to the same bias voltage. Since the deposition of different materials are optimized at different bias voltages, the targets for depositing one material have to be separated from the targets for depositing another material by at least the length of the pallet used. It follows that the manufacturing cost for depositing several layers of different materials increases significantly with the number of layers deposited.
The making of magnetic media used in conventional disc drives is one example. Conventional disc drives are used to magnetically record, store and retrieve digital data. Data is recorded to and retrieved from one or more discs that are rotated at more than one thousand revolutions per minute (rpm) by a motor. The data is recorded and retrieved from the discs by an array of vertically aligned read/write head assemblies, which are controllably moved from data track to data track by an actuator assembly.
The three major components making up a conventional hard disc drive are magnetic media, read/write head assemblies and motors. Magnetic media, which is used as a medium to magnetically store digital data, typically includes a layered structure, of which at least one of the layers is made of a magnetic material, such as CoCrPtB, having high coercivity and high remnant moment. The read/write head assemblies typically include a read sensor and a writing coil carried on an air bearing slider attached to an actuator. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. The actuator is used to move the heads from track to track and is of the type usually referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing closely adjacent to the outer diameter of the discs. Motors, which are used to spin the magnetic media at rates of higher than 10,000 revolutions per minute (rpm), typically include brushless direct current (DC) motors. The structure of disc drives is well known.
FIG. 1A illustrates a conventional magnetic media structure comprising a substrate 110, a nickel-phosphorous (NiP) layer 115, a seed layer 120, a magnetic layer 125 and a protective layer 130. The substrate 110 is typically made of aluminum or high quality glass having few defects. The nickel-phosphorous (NiP) layer 115 is an amorphous layer that is usually electrolessly plated or sputtered onto the substrate 110. The NiP layer is used to enhance both the mechanical performance and magnetic properties of the disk. The NiP layer enhances the mechanical properties of the disk by providing a hard surface on which to texture. The magnetic properties are enhanced by providing a textured surface that improves the magnetic properties including the orientation ratio (OR).
Seed layer 120 is typically a thin film made of chromium that is deposited onto the NiP layer 115 and forms the foundation for structures that are deposited on top of it. Magnetic layer 125, which is deposited on top of seed layer 120, typically includes a stack of several magnetic and non-magnetic layers. The magnetic layers are typically made out of magnetic alloys containing cobalt (Co), platinum (Pt) and chromium (Cr), whereas the non-magnetic layers are typically made out of metallic non-magnetic materials. Finally, protective overcoat 130 is a thin film typically made of carbon and hydrogen, which is deposited on top of the magnetic layers 125 using conventional thin film deposition techniques.
FIG. 1B is an illustration showing a front view of one side of a conventional magnetron sputtering system used to sputter deposit layers of the magnetic media. FIG. 1B shows a first target-cathodes 151, and a second target-cathode 152, both with erosion zones and redeposition areas, each located within a vacuum chamber 180 and 181 respectively. FIG. 1B also shows substrates 185, a pallet 187 a beam 191, a voltage bias power supply 160, a first power supplies 161, a second power supply 162, and a controller 165. Vacuum chamber 180 and 181 are conventional chambers, typically made of stainless steel that house target-cathodes 151 and 152 respectively, as well as a transport (not shown). Pallet 187 is typically made of aluminum and is machined to hold substrates 185 in an upward position and in an array. Beam 191 is typically a stainless steel beam from which pallet 187 hangs and is transported in vacuum chamber 180. The target-cathodes include both the target material to be sputtered, the cathode for applying a voltage to the target material, appropriate electrical connections, and cooling mechanism if needed along.
In FIG. 1B the target-cathodes are spaced apart so that first target-cathode 151 can be set at first voltage, second target-cathode 152 can be set at a second voltage, and pallet 187 can have a bias voltage set at different bias voltages for each of the target-cathodes. Pallet 187 is biased at a first bias voltage using the voltage bias power supply 160 and then transported in front of first target-cathode 151, which is set to voltage V1. Before the bias voltage can be set to a different level, pallet 187 must be transported completely passed first target-cathodes 151. Once the pallet 187 is moved passed first target-cathodes 151, the new and different bias voltage is set and the pallet is moved passed the second target-cathode 152, which is set to voltage V2. The pallet 187 must be moved completely passed first target-cathode 151 before the bias voltage can be changed because all of the substrates on the pallet are at the same bias voltage. Although using pallets to process multiple substrates is very efficient, there are disadvantages such as maintaining all substrates at the same process condition. For example, all the discs on the same pallet are set to the same bias voltage, although it may be advantage to have different parts of a pallet set at different bias voltages. This conventional means of applying different voltages is disadvantages because the pallet must be moved completely out passed the first two sets of target-cathodes before the bias voltage on the pallet can be changed. This translates to bigger sputtering systems that are more expensive to build and maintain.
In many applications it is advantageous to deposit different materials onto substrates that are biased at different voltages. Adjusting the substrate bias voltage according to the material being deposited optimizes thin film deposition processes. Since deposition processes are often optimized by adjusting the substrate bias voltage depending on the material to be deposited and all of the substrates on a pallet must be biased to the same voltage level, the target-cathodes for depositing one material have to be separated from the target-cathodes for depositing another material by at least the length of the pallet used.
These disadvantages become significant problems when depositing films in a manufacturing environment where throughput, costs, and floor space are major considerations. These problems are particularly important in the manufacture of disc drives because of the complexity of the magnetic media and the requirement that it be made inexpensively. The restriction of having to place target-cathodes far apart when using pass-by sputter tools significantly impacts the practicality of making magnetic media with pass-by sputter tools because complex magnetic media structures, as described with reference to FIG. 1, contain multiple layers that are often processed with different substrate bias voltages. Similar limitations exist on the use of pass-by sputter tools, as well as similar tools, in all industries that require depositing multiple layers on many substrates.
Therefore what is needed is a system and method that allows for variable biasing of a pallet loaded with discs, so that sputtering cathodes requiring different pallet biasing, do not have to be separated by at least the length of the pallet.